Tuesday, August 17, 2010

Continetal waltz and Plate boundaries

The continental waltz
     Using the residual magnetism of particles locked in various rocks, geologists can work out their orientation when the rocks were formed. By comparing their magnetic alignment, scientists can trace how the continents have moved. Two hundred million years ago almost all the continents were joined together in the super continent of Pangaea. By 100 million years ago, Pangaea had split into Laurasia and Gondwanaland which themselves began to split. 50 million years ago the Atlantic was just opening in the 50 million years time the face of the earth will have changes one more.
Plate boundaries
      Tectonic plates are ringed with destructive boundaries marked by earthquakes and volcanoes, Most new plate forms under the sea at mid ocean ridges. The ridges system is, in effect, the longest mountain chain on earth at 70,000 Km long. Crust produced along a line will not fit over the spherical surface of the Earth so the ridges buckle and are punctuated by 90 degrees transform faults.
      When an ocean plate meets another plate, it takes a dive at what is termed a subduction zone. This can result in an ocean trench fringed with volcanic atoll above the descending slab. Ocean crust sub ducting beneath a continent scrapes off a accumulated sediment and carries sea water as it goes. This results in volcanic activity and mountain building. When continent runs into continent, massive buckling occurs. Neither continental plate can sink so the rocks are uplifted into high mountain ranges.
     As larva from mid ocean ridges cools, it freezes the prevailing magnetism. Over time the Earth’s reversing magnetic field results in a striped pattern of magnetism either side of a ridge, a record of a sea floor spreading.